Fuel assemblies for a nuclear reactor, particularly fuel assemblies for a pressurized water nuclear reactor, are constituted by a bundle of elongate fuel elements termed fuel rods, arranged parallel to each other in the longitudinal direction of the assembly.
These fuel rods are constituted by tubes made of cladding material filled with fuel pellets.
The different fuel rods are held in position in the assembly by spacer plates and end plates connected to support tubes in place of some fuel rods and allowing the rigidity of the assembly to be assured.
During use in the core of the nuclear reactor, these assemblies can deteriorate under the action of various mechanical or thermal stresses or under the action of corrosion, so that the cladding material of the fuel rods can present fissures through which radio active material can pass into the cooling fluid of the reactor.
When the nuclear reactor is reloaded, during which operation used assemblies located in one part of the core are replaced by new assemblies, it is necessary to detect assemblies including leaking fuel rods.
During these operations, the core of the reactor is entirely immersed in a protective liquid such as water, and the assemblies involved in the reloading operations are conveyed under water from the vessel of the reactor to the fuel pond.
Leaking assemblies must be detected and these assemblies put in a region of the core in which reloading is taking place or in a region of the core in which assemblies are kept ready.
it is in fact essential to replace these leaking assemblies by new assemblies or to replace the defective rods inside the assembly by new rods.
When the assembly must definitely be replaced by a new assembly, it is still essential to know whether the used assembly is leaking, since, if this is the case, special precautions must be taken for moving or storing it.
To carry out the detection of a leaking assembly, the use of the apparatuses termed "crack detection cells" has been proposed, in which assemblies are disposed successively, one by one, inside the fuel pond. The temperature of the assembly is raised so that the pressure of the fission gases contained in the fuel rods increases and these gases escape inside the crack detection cell via the fissures in the rods, if these are defective.
Measurement of gamma activity in the fluid occupying the crack detection cell allows the escape of fission gas and hence the presence of a defective assembly, to be detected.
Such a crack detection cell is described in FRAMATOME French Pat. No. 2.389.202.
There is also a known method of detecting leaking fuel assemblies using sonic or ultrasonic acoustic phenomena connected with the impact of fission gas bubbles on a screen or with the build-up of the fission gases leaving the fuel rods through the faults in the cladding under the surface of the screen.
To implement this method, the gases must be expanded by heating or depressurizing of the assembly. These two methods of finding defective assemblies which are simple to implement and integrate perfectly with operations for transferring fuel allow defective assemblies to be determined during these transfer operations. But these methods do not allow the leaking fuel rod to be exactly located inside the assembly.
The defective fuel rods must obviously be identified when these rods are required to be replaced in the assembly by new rods before the assembly is reloaded in the core of the reactor.
Methods have therefore been proposed which allow defective fuel rods to be identified in assemblies during reloading operations on the nuclear reactor. For example, French Pat. No. 2.222.732, discloses detection of the presence of water in defective fuel rods by heating each of the rods of the assembly by induction and detecting bubbles of vapor or condensation which can occur at the cap of the fuel rod, by means of an ultrasonic echo test.
This method, which allows leaking fuel rods to be located, does necessitate partial dismantling of the assembly since each rod must be placed inside an apparatus for induction heating.
It has also been proposed, in French Pat. No. 2.287.753, to propagate an acoustic signal along the cladding of each of the fuel elements and pick up the signal obtained after propagation along the cladding of the fuel elements. When there is a defect in the cladding, an attenuation of the signal is observed, due to the presence of this defect.
To implement this method, an emitter and a receiver of acoustic waves must be placed on each rod, and the outer surface of the cladding of the fuel rod must be isolated from the cooling fluid, by placing the rod in a gaseous atmosphere.
Also, when the defect to be detected is at a distance from the end where the acoustic wave receiver is located, there is a risk of the return signal being drowned in the acoustic background noise.
There is therefore no known method allowing very reliable and easily implemented detection of defective fuel rods in an assembly which has not been dismantled.